1. Field of Invention
This invention relates to a radiation computerized tomography system, and more particularly to such a system wherein a data processing system is used for processing a flow of data in sequence between data collection and image reconstruction.
2. Description of Prior Art
Various computerized tomography (hereinafter referred to as "CT") systems have heretofore been known which employ radiation, such as X-rays, gamma rays or nuclear magnetic resonance (NMR) and electron beams (these rays and beams will herein generally and collectively be referred to as "radiation").
Also, various signal processing systems for such tomography systems are known, such as, for example, that disclosed in U.S. Pat. No. 4,135,247. Particular terminology of functions and components used therein may be found in this patent, and are applicable to the field of tomography in general.
FIG. 1(a) is a block diagram of a conventional X-ray CT system. As shown in FIG. 1(a), an X-ray source 1 irradiates a subject with a fan-shaped beam of X-rays, as shown by the dotted lines, and the X-rays, having passed through the subject 2, are detected by a detector 3 composed of an arcuate array of detecting elements curved about X-ray source 1. Each of the detecting elements generates an electric signal dependent on the intensity of the X-ray falling thereon. The electrical signals, or projection data, are read by a data acquisition system (hereinafter referred to as "DAS")4 for each view, and then converted into digital signals which are led to a central processing unit (hereinafter referred to as "CPU") 5. The CPU 5 transfers the data either directly, or after subjecting them to pre-processing, to a storage unit 6 (hereinafter referred to as "DISK") comprising a magnetic disk, for example. After data for all views have been transferred to DISK 6, a high speed processing unit (hereinafter referred to as "FP") 7 effects arithmetic operations of the projection data for purpose of image reconstruction. Then, a reconstructed image is displayed on a display unit (hereinafter referred to as "DISP") 8. With this system, however, as can be appreciated from study of the time diagram of FIG. 1(b), CPU 5 is occupied by data transfer process during scanning operation for successively storing output signal from DAS 4 into DISK 6. Thus, almost all of the image reconstruction processing is effected after scanning operation has been completed. The processed image data are stored in DISK 6 again and also transferred to DISP 8 where they are displayed. Thus, disadvantageously, it takes a long period of time for the conventional system to display the image after scanning has been completed.
Another system has been proposed, as shown in FIG. 2(a) to overcome the foregoing difficulty. With this proposed system a high speed processing unit FP 21 is interfaced with DAS 4 and is connected between DAS 4 and CPU 5 for purpose of simplifying data flow during scanning operation. But, as can be appreciated from study of the time chart of FIG. 2(b), because of a program in FP 21 for reading data from DAS 4, the process which FP 21 can perform during scanning operation only includes pre-processing and convolution on data from DAS 4 for each view. Disadvantageously, back projection is effected only after scanning operation has been completed. This system is advantageous in that the time required until an image can be displayed is reduced since data processing after the scanning cycle is of a lesser extent than that of the above described system. However, disadvantageously, there is idle time while waiting for a next series of view data in the scanning operation, as can be seen in FIG. 2(b).
FIG. 3(a) depicts another prior system, which includes two high speed processing units (FP1 and FP2) 31,32. Front FP1 31 interfaces with DAS4 and effects preprocessing and convolution, while rear FP2 32 connected between FP1 31 and CPU 5, performs back projection. Data processed by FP1 31 are delivered in a pipelining fashion to FP2 32 in synchronism with each view, and the data is subjected in FP2 32 to back projection processing for each view.
Thus, image reconstruction is completed substantially at the same time that a scanning cycle is brought to an end, as shown in the time diagram of FIG. 3(b). Although the system of FIG. 3(a) can greatly reduce the time required for image reconstruction, disadvantageously, it is highly expensive, since two expensive high speed processing units (FP) are required.
Also, computation for image reconstruction is usually performed in a processing unit having a longer bit length than that of raw data in order to maintain a required degree of accuracy. For example, the processing unit effects a 32 bit integer or floating point arithmetic operation on raw data comprising 16 bit integer. Thus, the image reconstruction apparatus has processing and memory units capable of handle data of long bit lengths. Where raw data are delivered into the memory unit in the image reconstruction apparatus by way of direct memory access (herein referred to as "DMA"), the operation can be effected at higher speed. However, the memory has idle storage locations since it only receives raw data of short bit length.